The invention relates to buck-boost switched voltage regulators (also referred to as buck-boost DC--DC voltage converters), and more particularly to improvements therein which reduce power dissipation, provide smooth transitions between buck mode and boost mode, and provide stable operation during fast switching of the load current.
So-called Cuk and Sepic switching topologies are known for buck-boost voltage regulation, and are described in "Dynamic Analysis of the Switch-Mode DC/DC Converters", by Andre Kislovski et al., Van Nostrand Reinold, New York, pp. 177-203. The Cuk and Sepic topologies include two inductors. This nearly doubles the amount of circuit board area, the cost, and the amount of EMI (electromagnetic interference) produced by the regulator. Another typical prior art buck-boost switching voltage regulator includes two power switches, two diodes (or synchronous rectifiers), one inductor, and a load capacitor. Such typical prior art buck-boost switching voltage regulators are described in U.S. Pat. Nos. 4,395,675, 4,578,630, 5,528,125, 5,734,258 and 5,602,463. These buck-boost topologies typically include two simultaneously switching power transistors and two diodes or synchronous rectifiers operating invert each clock or oscillation cycle. This essentially doubles the power dissipation of the regulator in most of its operating modes.
U.S. Pat. No. 5,734,258 discloses complex inductor coupling arrangements in a buck-boost switching regulator, wherein switching decisions are based on the relationship between the unregulated input voltage and the output voltage. This technique does not allow fast response to rapid changes in the load. Furthermore, the operation of circuitry within the buck-boost switching regulator is unpredictable when the output voltage is nearly equal to the unregulated input voltage. Furthermore, products using complex custom inductor arrangements and transformer arrangements are unsuitable for high-volume production.
Prior buck-boost switched voltage regulators typically use a comparator to determine whether the input voltage or the regulated output voltage is larger, and use that information to control only one of the switches. This approach is often slow because too much "cycling" is required. Furthermore, if the unregulated input voltage goes significantly below the regulated output voltage, that may cause serious circuit stability problems.
Therefore, there is an unmet need for a low-cost, fast-responding integrated circuit buck-boost switching voltage regulator having stable circuit operation despite large differences between the unregulated input voltage and the regulated output voltage.